Voltage Drop....G Cable

[bwaggener]

What is the voltage drop on a 1/0- 3 conductor type G cable on a 600' feeder @ 240 volt, single phase with a 200 amp load?

 

[roger]

AFIK, 1/0 type G is not rated for 200 amps. Is this a test question / homework?

Roger

 

[don_resqcapt19]

Rated at 181 in the 75°C column if Table 400.5(A)(2), so cannot be used for a 200 amp load.
There is also an issue with using any of the cables in that table at the table ampacities because 110.14 says we can only use the Table 310.15(B)(16) ampacities. That table is 310.16 in the 2020 code.

 

[bwaggener]

AFIK, 1/0 type G is not rated for 200 amps. Is this a test question / homework?

Roger

Actually it is rated for 207 amps in a single phase case.

 

[bwaggener]

Rated at 181 in the 75°C column if Table 400.5(A)(2), so cannot be used for a 200 amp load.
There is also an issue with using any of the cables in that table at the table ampacities because 110.14 says we can only use the Table 310.15(B)(16) ampacities. That table is 310.16 in the 2020 code.

Column "E" is used in a single phase case like this. Which is rated at 207 amps.

 

[bwaggener]

The issue here is that the job is designed to use G-GC cable. (3 conductor cable for single phase circuits).
The design engineer is using THHN for his voltage drop calculations.
The circuits are very long, some near 600'. So voltage drop is a concern.
And, using THHN, @ 600' with a 200 amp load will have a significant VD.
According to Holts VD calculator, I would need a 300 mcm to get below 5%.
G cable has substantially higher current ratings than THHN or other normal wire.
The engineer on the job states that because of that the G cable also has a higher amp rating, it is also more tolerant regarding VD.
And he is using the much smaller G cable at face value on these long circuits disregarding the VD.
It's troubling to me, thus my post.

 

[tom baker]

The volt drop creates heat in the cable, other than that it doesn't care.
But the load may not like the volt drop.

 

[zbang]

G cable has substantially higher current ratings than THHN or other normal wire.

Current ratings are based on the ability of the insulation to withstand heat. (That's why we do VD calcs based on the resistance/size, not the insulation type.)

The engineer on the job states that because of that the G cable also has a higher amp rating, it is also more tolerant regarding VD.

It's more tolerant, but that's irrelevant- think of the cable as a resistor and one type has a higher power rating than another.

I will only mention termination temperatures in passing.....

 

[don_resqcapt19]

The voltage drop is based on the amount of copper in the conductor and not the amp rating of the conductor. The conductor does not care about voltage drop unless the drop is so excessive that the heat damages the insulation of the conductor. Only the load cares about the voltage drop. The engineer is 100% wrong saying that the type of conductor or cable makes a difference in how much voltage drop is permitted for a given circuit.

 

[don_resqcapt19]

Column "E" is used in a single phase case like this. Which is rated at 207 amps.

I did skip over the use of column E. However, I still see no permission in the code that permits you to uses that cable at that ampacity when directly connected to any type of equipment.

 

[winnie]

Next question: how to get the engineer to reconsider this point without making it a fight.

I agree with the above comments; no permission in the code to use this cable at this amp rating, and voltage drop will be no better than THHN of the same gauge and length.

One possible voltage drop calc issue: you say '200A', however is the load really 200A, or is it a circuit protected by a 200A breaker? They are two different things. If you have a circuit protected by a 200A breaker, the actual load might be less than 200A...or if the load has significant starting current then you might see even more than 200A for short periods of time. You need to use the actual current flow to calculate voltage drop.

-Jon

 

[bwaggener]

Next question: how to get the engineer to reconsider this point without making it a fight.

I agree with the above comments; no permission in the code to use this cable at this amp rating, and voltage drop will be no better than THHN of the same gauge and length.

One possible voltage drop calc issue: you say '200A', however is the load really 200A, or is it a circuit protected by a 200A breaker? They are two different things. If you have a circuit protected by a 200A breaker, the actual load might be less than 200A...or if the load has significant starting current then you might see even more than 200A for short periods of time. You need to use the actual current flow to calculate voltage drop.

-Jon

The 200 amp load in the example is an example of an actual connected load.
In this particular case the load is power pedestals on floating docks at a marina.
Two pedestals with (2) 50 amp, 240 volt receptacles each.
Regarding the insulation reply above, I expressed that exact argument to the engineer.
I told him that I have always understood current ratings of conductors to be directly related to the insulation type.
So I agree still with that.
His response was regarding the fact that G cable is highly stranded.
That current travels on the surface of the conductors and that is why the G cable is so capable of both higher current ratings and better VD tolerances.
He went on to say that he has proven it thru testing.
So I am an electrical contractor, not an engineer.
I've been in business since 1981 and I have never heard the argument made by this engineer in all these years.
The NEC looks only at circular mills, load and distance to determine VD.
As far as I know, there are no exceptions to this regarding fine stranding of the conductor.

That is why I posed the question above.
I have the same question posed to the engineers at General Cable and Southwire.
They manufacture the stuff, hopefully they will provide a definitive answer to my question.

 

[roger]

Besides the VD and ampere rating issues, 400.7 and 400.8 will come into play.

Roger

 

[winnie]

The 200 amp load in the example is an example of an actual connected load.
In this particular case the load is power pedestals on floating docks at a marina.
Two pedestals with (2) 50 amp, 240 volt receptacles each.

So a 50A receptacle is not likely to be loaded to the full 50A, and with 4 boats sharing a 200A circuit load diversity will come into play. So that 200A circuit is likely not loaded to 200A. However if one boat has a large motor load using a big chunk of that 50A, then every time it starts everyone on that same feeder will experience a large transient voltage drop.

Regarding the insulation reply above, I expressed that exact argument to the engineer.
I told him that I have always understood current ratings of conductors to be directly related to the insulation type.
So I agree still with that.
His response was regarding the fact that G cable is highly stranded.
That current travels on the surface of the conductors and that is why the G cable is so capable of both higher current ratings and better VD tolerances.
He went on to say that he has proven it thru testing.

Frankly I want to buy whatever it is the engineer is smoking. bwaggener you are spot on.

-Jon

 

[zbang]

His response was regarding the fact that G cable is highly stranded.
That current travels on the surface of the conductors and that is why the G cable is so capable of both higher current ratings and better VD tolerances.

He doesn't have a clue about the skin effect, which at 60Hz is something like 8mm (1/3 inch).

He's also an idiot (IMHO).

 

[ramsy]

Rated at 181 in the 75°C column if Table 400.5(A)(2), so cannot be used for a 200 amp load. (or if only 2ccc, 207A in column E)
There is also an issue with using any of the cables in that table at the table ampacities because 110.14 says we can only use the Table 310.15(B)(16) ampacities. That table is 310.16 in the 2020 code.

Amazed anyone found this table 400.5(A)(2), with cables operating at such high temperatures.

Don points out 110.14(C)(1) instruction shows 200A on 1/0cu may be ~113°C, which far exceeds 75°C equip. maximums.

Per Chap.9, Tbl 8, Note 2, 200A operates on 1/0cu @ 113°C.
Per Chap.9, Tbl 9, Note 2, 600ft of this @ 240 volts drops to 207 volts, and 120v drops to 87v.
Switch mode power supply nameplates, Computers, TV's, etc, show a 100vac minimum.

A vault transition to 10ft of 3/0cu pigtails at equip. termination may bring 195A down to 75°C, but voltage drop remains problematic ~14%.

 

[AKElectrician]

Next question: how to get the engineer to reconsider this point without making it a fight.
-Jon

Have him install it.
Cheaper to do nothing, than something wrong.
Unless you work for a oil company, then it costs you at the pump. So charge double you might break even.

 

[topgone]

Amazed anyone found this table 400.5(A)(2), with cables operating at such high temperatures.

Don points out 110.14(C)(1) instruction shows 200A on 1/0cu may be ~113°C, which far exceeds 75°C equip. maximums.

Per Chap.9, Tbl 8, Note 2, 200A operates on 1/0cu @ 113°C.
Per Chap.9, Tbl 9, Note 2, 600ft of this @ 240 volts drops to 207 volts, and 120v drops to 87v.
Switch mode power supply nameplates, Computers, TV's, etc, show a 100vac minimum.

A vault transition to 10ft of 3/0cu pigtails at equip. termination may bring 195A down to 75°C, but voltage drop remains problematic ~14%.

It's late chiming in but maybe the engineer looked at the instructions at the foot of the table 400.59A2)! Since the type-G cable was 3-conductor, the correct rating of the conductor is 181A (column F, 75 deg), not 207A as pointed out!

 

[ramsy]

See post #3